Replies

1. 
   rhl_ | May 12, 2019 08:05am | #1

   Is this on the floor or in the joist bays? You need to worry about moisture control. In either location if the fluffy stuff goes inside the bay, and then the rigid stuff goes over the studs, that will be a problem.

   In an idealized world, you can reverse the order, putting the rigid stuff inside the bays first, then backfilling the rest with fluffy stuff. This is called cut and cobble [+ batt]. The trouble with this approach is that it can be hard to get a good air seal. Even it it works out at install time, the house can shift, and the air seal can break. (this is one reason why exterior insulation is preferred).

   If this is on the attic floor, consider dense pack cellulose.

2. 
   RaWe | May 12, 2019 08:52am | #2

   Thank you. I was afraid the get this answer. Will stick with just stone wool.

3. GBA Editor
   Martin Holladay | May 13, 2019 06:35am | #3

   RaWe,
   I agree with Ryan. Just add more mineral wool if you want more insulation.

4. 
   Peter Yost | May 14, 2019 05:46am | #4

   Does your ceiling/attic insulation plan have an air barrier? It needs one.

   I don't understand why installing rigid insulation to the exterior of the cavity insulation would pose a problem; we install continuous rigid insulation on the exterior of wall assemblies all the time, to warm the air-permeable insulation to the interior and also if we seal those exterior insulation panels, we achieve an air barrier. So long as the "exterior" (topside) rigid insulation has sufficient R-value based on the R-value of the cavity insulation for the climate, should work fine.

   I am not disagreeing with the economics of just adding more loose fill insulation. With an air barrier, that solution makes the most sense but I want to make sure I understand the problem the proposed rigid insulation solution poses.

   Peter

1. GBA Editor
   Martin Holladay | May 14, 2019 06:09am | #5

   Peter,
   If RaWe is talking about horizontal rigid foam above mineral wool batts on an attic floor, the two main problems I'm thinking of are: (1) it will be impossible for maintenance workers or electricians to walk on the rigid foam without damaging it, and (2) taping the seams of the rigid foam will be a real pain.

1. 
   RaWe | May 14, 2019 03:41pm | #6

   Yes, that's why I stick with a double layer of Rockwool.
   Thanks to all for your inputs.